Regulation of the KstR2 regulon of Mycobacterium tuberculosis by a cholesterol catabolite.

Cholesterol catabolism is widespread in actinobacteria and is critical for Mycobacterium tuberculosis (Mtb) virulence. Catabolism of steroid nucleus rings C and D is poorly understood: it is initiated by the CoA thioesterification of 3aα-H-4α(3'-propanoate)-7aβ-methylhexahydro-1,5-indanedione (HIP) by FadD3, whose gene is part of the KstR2 regulon. In Mtb, genes of this regulon were upregulated up to 30- and 22-fold during growth on cholesterol and HIP, respectively, versus another minimal medium. In contrast, genes involved in degrading the cholesterol side-chain and nucleus rings A and B were only upregulated during growth on cholesterol. Similar results were obtained in Rhodococcus jostii RHA1. Moreover, the regulon was not upregulated in a ΔfadD3 mutant unable to produce HIP-CoA. In electrophoretic mobility shift assays, HIP-CoA relieved the binding of KstR2(Mtb) to each of three KstR2 boxes: CoASH, HIP and a related CoA thioester did not. Inspection of the structure of KstR2(RHA1) revealed no obvious HIP-CoA binding pocket. The results establish that Mtb can catabolize the entire cholesterol molecule and that HIP-CoA is an effector of KstR2. They further indicate that KstR2 specifically represses the expression of the HIP degradation genes in actinobacteria, which encode a lower pathway involved in the catabolism of multiple steroids.